The invention relates to a capacitive humidity sensor comprising a substrate being a flexible copper laminate, a moisture-impermeable conducting bottom layer etched in said copper laminate as first capacitor plate, a dielectric layer of which the dielectric constant is a function of moisture absorption, and a moisture-permeable conducting top layer as second capacitor plate, said bottom layer and a contact layer making contact with the moisture-permeable top layer and being deposited adjacent each other and isolated from each other on the substrate, the top layer extending both over the dielectric layer and over the contact layer. Such a humidity sensor is disclosed in European patent application 0094266.
In such a sensor, which in fact consists of an electrical capacitor, the humidity is determined by measuring the change in dielectric constant of the dielectric medium, and therefore the change in capacitance value, due to the absorption of water molecules by the dielectric material. If, for example, the material of a dry dielectric layer has a dielectric constant of 3.5 and water approximately has one of 80, it will be clear that the change in capacitance value due to moisture absorption may be appreciable. In order that moisture or water can be absorbed by the dielectric layer, the top layer has to be moisture-permeable.
This known sensor has a substrate on which a copper layer as first capacitor plate is deposited. A moisture-impermeable insulating layer is provided on top of the first capacitor plate. Said insulating layer serves as a barrier layer and prevents resistance loss across the dielectric medium at high relative humidity, or even short circuiting between the plates regardless of the structure of the dielectric medium. The dielectric layer mentioned is sufficiently porous to absorb moisture.
Now in practice it appears that the use of such a sensor is strictly dependent on well-defined precalibration techniques under dry and moist atmospheric conditions before using it for the consumer market and for industrial application.
The object of the invention is to eliminate the above mentioned problem and to provide a cheap sensor which is most robust and which can be easily calibrated after manufacture.